Re: makefs alignment issue

Dave Chinner
2014-10-28T00:33:33Z

On Mon, Oct 27, 2014 at 06:04:05PM -0500, Stan Hoeppner wrote:
> On 10/26/2014 06:43 PM, Dave Chinner wrote:
> > On Sat, Oct 25, 2014 at 12:35:17PM -0500, Stan Hoeppner wrote:
> >> If the same interface is used for Linux logical block devices (md, dm,
> >> lvm, etc) and hardware RAID, I have a hunch it may be better to
> >> determine that, if possible, before doing anything with these values.
> >> As you said previously, and I agree 100%, a lot of RAID vendors don't
> >> export meaningful information here. In this specific case, I think the
> >> RAID engineers are exporting a value, 1 MB, that works best for their
> >> cache management, or some other path in their firmware. They're
> >> concerned with host interface xfer into the controller, not the IOs on
> >> the back end to the disks. They don't see this as an end-to-end deal.
> >> In fact, I'd guess most of these folks see their device as performing
> >> magic, and it doesn't matter what comes in or goes out either end.
> >> "We'll take care of it."
> >
> > Deja vu. This is an isochronous RAID array you are having trouble
> > with, isn't it?
>
> I don't believe so. I'm pretty sure the parity rotates; i.e. standard
> RAID5/6.

The location of parity doesn't dtermine that it is isochronous in
behaviour or not. Often RAID5/6 is marketing speak for "single/dual
parity", not the type of redundancy that is implemented in the
hardware ;)

> > FWIW, do your problems go away when you make you hardware LUN width
> > a multiple of the cache segment size?
>
> Hadn't tried it. And I don't have the opportunity now as my contract
> has ended. However the problems we were having weren't related to
> controller issues but excessive seeking. I mentioned this in that
> (rather lengthy) previous reply.

Right, but if you had a 768k stripe width and a 1MB cache segment
size, a cache segment operation would require two stripe widths to
be operated on, and only one would be a whole stripe width. hence
the possibility of doing more IOs than are necessary to populate
or write back cache segments. i.e. it's a potential reason for
why the back end disks didn't have anywhere near the expected seek
capability they were supposed to have....

> >> optimal_io_size. I'm guessing this has different meaning for different
> >> folks. You say optimal_io_size is the same as RAID width. Apply that
> >> to this case:
> >>
> >> hardware RAID 60 LUN, 4 arrays
> >> 16+2 RAID6, 256 KB stripe unit, 4096 KB stripe width
> >> 16 MB LUN stripe width
> >> optimal_io_size = 16 MB
> >>
> >> Is that an appropriate value for optimal_io_size even if this is the
> >> RAID width? I'm not saying it isn't. I don't know. I don't know what
> >> other layers of the Linux and RAID firmware stacks are affected by this,
> >> nor how they're affected.
> >
> > yup, i'd expect minimum = 4MB (i.e stripe unit 4MB so we align to
> > the underlying RAID6 luns) and optimal = 16MB for the stripe width
> > (and so with swalloc we align to the first lun in the RAID0).
>
> At minimum 4MB how does that affect journal writes which will be much
> smaller, especially with a large file streaming workload, for which this
> setup is appropriate? Isn't the minimum a hard setting? I.e. we can
> never do an IO less than 4MB? Do other layers of the stack use this
> variable? Are they expecting values this large?

No, "minimum_io_size" is for "minimum *efficient* IO size" not the
smallest supported IO size. The smallest supported IO sizes and
atomic IO sizes are defined by hw_sector_size,
physical_block_size and logical_block_size.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com